The present invention relates to an imaging system, particularly, to a negative-to-positive conversion apparatus for television signals, in which signals representative of a negative image as recorded on a negative recording medium, such as a color negative film, are converted into television signals representative of a positive image associated therewith.
Where a negative image recorded on a color negative is imaged by an imaging device, such as a color television camera, in order to produce a positive image associated therewith, it was common in the prior art that signals developed from the imaging device and representative of a negative color image are separated into separate color signals, which are in turn inverted in color and brightness to form a positive image associated therewith.
Recent developments in television cameras are directed to inclusion of integrated circuitry for of compactness and power savings. It is common in commercially available, integrated circuit color telelvision cameras that the integrated circuit chip involved therein is so designed as to produce luminance, Y, signals, and color difference signals, such as B-Y and R-Y, where B and R represents blue and red signals, respectively, rather than separated color signals, such as red, green (G) and blue.
For negative-to-positive conversion by means of luminance and color difference signals, in general, there are conventionally three techniques used: one is respective inversion made on those luminance and color difference signals; another is inversion of luminance signals with 180 angular degrees of shift in phase made on color subcarriers of composite color video signals; and the other is inversion of luminance signals with 180 angular degrees of shift in phase made on a color burst of composite color video signals. Those techniques are, however, disadvantageous in the provision of sophisticated circuitry required for accomplishing color correction on video signals representative of a negative image.
In general, color negative film is exposed under various lighting conditions with less correction made on color temperature during exposure. It is therefore necessary to correct color temperature on the images under negative-to-positive conversion.
In order to accomplish color temperature correction under a variety of lighting conditions, it is required to make such correction on at least two sorts of color components resolved from the images in question. Conventionally, there is used a color temperature correction system including a color monitor device, such as a cathode-ray tube display, displaying thereon an image, and two series of knobs, red and blue, and green and purple, which knobs are manipulated by an operator so as to control color temperature correction while viewing an image indicated on the display. It is however, remarkably difficult for the operator to control two series of manipulating knobs while viewing the display screen on which a white sample image is visualized. This results from the fact that it is difficult to adjust two independent components defined on an orthogonal plane, in other words, one cannot visually identify the origin of color vectors with respect to each of red and blue color vectors, and green and purple color vectors. Accurate adjustment is therefore accomplished by means of measurement devices, such as a vectorscope. It is however not suitable to incorporate a negative-to-positive conversion apparatus including such a measurement device, requiring accurate manual adjustment, into a television camera which is designed to be suitable for home use.
A prior art telecine system for studio use is usually loaded with film which was exposed under controlled, or standard lighting conditions, rather facilitating white balance correction. With a white balance adjustment required in a negative-to-positive conversion apparatus, however, situations are much more complicated in that an image recorded on a negative film under uncontrolled lighting conditions, such as at home, is picked up by a separate image sensor under different lighting conditions to develop a television picture.
Anyway, strictly speaking, it is impossible to reproduce in detail the visual information presented by an original scene only from the limited information carried on the negative film. In the case of a telecine system for use in a broadcasting station, as discussed above, technicians skilled in correction and adjustment of television pictures imaged by a television camera may often have the pictures displayed on a picture monitor device with appropriate corrections made as to color, balance and brightness, and to highlight and shadow portions thereof, so as to be observed as in a natural condition, only by their knowledge of the limited information carried on the negative film with respect to the lighting conditions used during the exposure.
It would however hardly be expected in reproduction of images by a home-use system to have an operator perform such sophisticated operations as to require their experience on adjustment and correction of color and white balance of images to be incorporated thereinto. Difficulty in system configuration and cost is also involved in providing such a home-use system with an accurate manipulation of the color and white balance adjustment mechanism, as well as a measuring arrangement for color component vectors.
Commonly available is the type of color negative film which has a broader latitude, effecting images to be appropriately recorded thereon under a variety of exposure values with wider deviations in color balance from the standard.
In order to produce, from such a variety of images recorded on the negative film, color prints carrying positive images associated therewith, specific difficulties as discussed above do not arise since the negative film is dealt with in commercial laboratories with color and white balance corrections made by the skilled technical staff. In applying such film to a television picture reproduction system, it was very difficult to accomplish brightness and color corrections, or, at least white balance correction per se, on such a variety of images carried on the film. Variations in exposure amount may be corrected to a certain extent by making adjustment on an optical aperture of the image sensor involved in the negative-to-positive conversion system. Some kinds of subject images recorded on film, an image recorded with a larger difference in brightness through measuring the brightness of the entire field of the camera view, may make it difficult to achieve complete corrections only by means of aperture controlling.